Heat treat furnaces of the type herein contemplated generally are constructed with an exterior metal shell lined with solid refractory insulating bricks or blocks. The furnaces typically operate at temperatures in excess of about 1200.degree. F. and an appreciable thickness of insulating block is necessary to avoid excessive heat losses through the furnace walls. For example, in a furnace operating at 1800.degree. F. in an ambient temperature of 80.degree. F., a structural wall made up of 9 inches of insulating fire brick (K-23 brick, manufactured by Babcock & Wilcox) and two inches of block insulation (Johns-Manville Superex M) has a heat loss per square foot of wall area of 310 B.T.U., the exterior metal wall temperature of 213.degree. F., and the interface temperature between the insulating fire brick and the block wall will be 660.degree. F. The cost of the heat loss and fuel consumption in a large furnace and the ambient air heating effect from the exterior wall temperature of 230.degree. F. will be readily appreciated.
It has been proposed, as in U.S. Pat. No. 2,828,813 to A. F. Holden, that a furnace be lined with permeable fire brick spaced from the exterior metallic casing to define a plenum chamber. A combustible gas-air mixture is introduced into the plenum chamber. The combustible mixture burns at the interior lining surface of the furnace after it has been pre-heated by passing through the permeable fire brick lining. Such luminous wall furnaces have been utilized commercially, albeit for limited purposes due to the restriction placed upon the atmospheres with which it may be utilized.